Add an extra transition type in TransitSession for custom transition.

In TransitionUtil#setupLeash, there will check whether the transition
belong to opening type, and adjust the layer order according it. Which
cannot support for custom type.
By adding an extra transition info in TransitSession, it can help
identify custom transition, without create new type of transition.

Bug: 279104525
Test: atest SplitTransitionTests
Test: verify the opening tasks remain visible during opening animation.
Change-Id: Ic1302e211bc94ec66a293518b3823686e50e1ab2